542 GENERAL CONSIDERATIONS 



system was inadequate and so there was great confusion in the classifica- 

 tion and naming of animals. A step forward, however, was taken by 

 Ray (1628-1705), who first fixed a definite conception of a species and 

 also used anatomical facts in the discrimination of the larger groups. 

 His work paved the way for that of Linnaeus (1707-1778). Linnaeus 

 (Fig. 328) believed in the fixity of species and his classification was dis- 

 tinctly artificial, but it is so simple in theory, so clear, and so plastic 

 that it has furnished the basis of all the work in this field which has been 

 done since. He divided animals into six classes (four of which were 

 vertebrate groups), 32 orders, and a large number of genera and species. 

 He also gave a brief characterization in Latin of each higher group and of 

 each species. While they were sufficient for the limited number of groups 

 and species recognized then, these characterizations are not sufficiently 

 detailed or exact in the light of our present knowledge; however, they 

 served as models for those who followed him. It is to be observed, more- 

 over, that his groups were of much higher rank than the groups given 

 the same names today, his classes being equivalent to present-day phyla, 

 his orders to what are now classes, and his genera to orders as now recog- 

 nized. While we use his generic names we have greatly limited the 

 extent of his genera. Linnaeus believed in the possibility of arranging 

 animals in a single series. To Lamarck (1744-1829) is to be given credit 

 for first recognizing the fact that there are different lines of descent in the 

 animal kingdom and for representing this in the form of what has since 

 been known as a genealogical, or phylogenetic, tree. The effect of the 

 general acceptance of the evolutionary conception was to cause a change 

 toward a more natural system of classification. In 1866, Haeckel pre- 

 sented the first really modern classification. 



615. Species. — While to most people a species seems to be a very 

 definite conception, insuperable difficulties are immediately encountered 

 when an attempt is made to define it. Although many criteria have been 

 used in the definition of species, it is still a matter of opinion as to whether 

 a species is a natural entity or an artificial assemblage of individuals 

 and as to what terms shall be used in the definition of the word. An 

 assemblage of individuals which to one zoologist appears clearly separable 

 into a number of species may be considered by another as simply repre- 

 senting the variations possible in one. The extent of any taxonomic 

 group is to a considerable degree a matter of opinion. Generally speak- 

 ing, the differences between species are qualitative. If the differences 

 between two forms are only quantitative, such as differences in size and 

 shade of color, the usual conclusion is that they are only forms of one 

 species. Some species are very conservative, exhibiting Httle variation 

 throughout their range; others are radical, varying with every change in 

 the environment. 



